1. Field of the Invention
The present invention relates to device and method for detecting a state of liquid droplets discharged from nozzle holes of a recording head as well as to an image forming device such as a printer, copier, or facsimile machine incorporating such a device.
2. Description of the Related Art
In general a serial type image forming device and a line type image forming device are known. The serial-type image forming device forms an image by a recording head's discharging liquid droplets while moving in a main scanning direction. The line-type image forming device uses a line head to discharge liquid droplets without moving. Because of forming images by discharging liquid droplets, the image quality of these image forming devices deteriorates if the level of nozzle discharge is degraded due to an increase in ink viscosity and ink solidification arising from evaporation of solvent from nozzles, dust attachment, bubble interfusion or else.
In view of this, Japanese Patent No. 4925184 discloses a device for detecting the state of liquid droplets discharged from a recording head, for example. This device is a forward scattered light type which emits a laser beam from one side of a nozzle row of a recording head along the nozzle row and receives scattered light from the liquid droplets on a light receiving element placed on the other side of the nozzle row aside from the optical axis of the light beam, to thereby determine presence or absence of a discharged liquid droplet. In this document a single light receiving element is provided for one or more nozzle rows.
Further, aiming for accurately detecting a state of discharged fluid such as ink droplets with a less complicated structure at low cost, the above document discloses, for example, a structure that light receiving elements are disposed at two more separate locations off the optical axis of a light beam from the light emitting element, to be able to receive scattered light and detect an ink discharged state from a difference in the optical output values of the light receiving elements.
However, the related-art technique disclosed in the above document has a problem that when the optical axis of the light beam is tilted toward the light receiving elements and made aligned with liquid droplets closer to the light emitting element for detection, the optical axis becomes closer to the light receiving elements positioned as above so that amount of offset light (noise light) including a part of the light beam or reflected light is increased. Due to the increase in the offset light amount, the received light amount exceeds a saturation limit value of a detection circuit, making it difficult to detect a state of discharged liquid droplets. Meanwhile, when the optical axis of the light beam is tilted to the opposite side of the light receiving elements and made aligned with liquid droplets closer to the light receiving elements for detection, the optical axis becomes further from the light receiving elements positioned as above. Because of this, the amount of received scattered light does not reach a detectable threshold, making it difficult to detect a state of discharged liquid droplets.